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                <h2 id="事件机制"><a href="#事件机制" class="headerlink" title="事件机制"></a>事件机制</h2><blockquote>
<p>事件的触发过程是怎么样的?什么是事件代理?</p>
</blockquote>
<h3 id="事件触发三阶段"><a href="#事件触发三阶段" class="headerlink" title="事件触发三阶段"></a>事件触发三阶段</h3><ol>
<li>捕获阶段<ul>
<li><code>window</code>往事件触发处传播(上层传到底层),遇到注册的捕获事件会触发</li>
</ul>
</li>
<li>目标阶段<ul>
<li>事件到达目标节点时,就到了目标阶段,事件在目标节点上被触发</li>
</ul>
</li>
<li>冒泡阶段<ul>
<li>从事件触发处往<code>window</code>传播(底层传回上层),遇到注册的冒泡事件会触发</li>
</ul>
</li>
</ol>
<p>事件触发一般会按照上面的顺序进行,但也有特例,<strong>如果给<code>body</code>中的子节点同时注册冒泡和捕获事件,事件触发会按照注册的顺序执行</strong></p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 以下会先打印冒泡然后打印捕获</span></span><br><span class="line">node.addEventListener(</span><br><span class="line">  <span class="string">&#x27;click&#x27;</span>,</span><br><span class="line">  event =&gt; &#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;冒泡&#x27;</span>)</span><br><span class="line">  &#125;,</span><br><span class="line">  <span class="literal">false</span></span><br><span class="line">)</span><br><span class="line">node.addEventListener(</span><br><span class="line">  <span class="string">&#x27;click&#x27;</span>,</span><br><span class="line">  event =&gt; &#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;捕获&#x27;</span>)</span><br><span class="line">  &#125;,</span><br><span class="line">  <span class="literal">true</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure>
<h3 id="注册事件"><a href="#注册事件" class="headerlink" title="注册事件"></a>注册事件</h3><p>通常使用<code>addEventListener</code>注册事件,该函数的第三个参数可以是布尔值,也可以是对象。对于布尔值<code>useCapture</code>参数来说,该参数默认值为<code>false(冒泡)</code>,<code>useCapture</code>决定了注册的事件是<code>捕获事件</code>还是<code>冒泡事件</code>。对于<code>对象参数</code>来说,可以使用以下几个属性</p>
<ul>
<li><code>capture</code>: 布尔值,和<code>useCapture</code>作用一样</li>
<li><code>once</code>: 布尔值,值为<code>true</code>表示该回调只会调用一次,调用后会移除监听</li>
<li><code>passive</code>: 布尔值,表示永远不会调用<code>preventDefault(阻止元素发生默认行为)</code></li>
</ul>
<p>一般来说,如果只希望事件只触发在目标上,这时候可以使用<code>stopPropagation</code>来阻止事件的进一步传播。<code>stopPropagation</code>是用来阻止事件冒泡的,<code>stopImmediatePropagation</code>是用来阻止事件冒泡并且阻止该元素上同事件类型的监听器被触发</p>
<h3 id="事件代理"><a href="#事件代理" class="headerlink" title="事件代理"></a>事件代理</h3><p>如果一个节点中的子节点是动态生成的,那么子节点需要注册事件的话<strong>应该注册到父节点上</strong></p>
<p><code>事件代理</code>的本质就是利用<code>事件冒泡</code>的机制把里层所需要响应的事件绑定到外层</p>
<p>事件代理的方式相比较于直接给目标注册事件来说,有以下优点:</p>
<ul>
<li>节省内存空间,减少事件注册,提高性能</li>
<li>如果有新增元素,不用再重新给父元素绑定事件,实现了动态绑定事件</li>
</ul>
<h2 id="跨域"><a href="#跨域" class="headerlink" title="跨域"></a>跨域</h2><blockquote>
<p>什么是跨域?为什么浏览器要使用同源策略?有哪几种方式可以解决跨域问题?</p>
</blockquote>
<p>因为浏览器出于安全考虑,有<code>同源策略</code>。也就是说,如果<code>协议</code>、<code>域名</code>、<code>端口号</code>有一个不同就是跨域,Ajax请求会失败</p>
<p><strong>那么是出于什么安全考虑才会引入这种机制?</strong><br>其实主要是用来防止CSRF攻击的。简单来说,CSRF攻击是利用用户的登录状态发起恶意请求。</p>
<p>也就是说,没有同源策略的情况下,A网站可以被任意其他来源的Ajax访问到内容。如果你当前A网站还存在登录状态,那么对方就可以通过Ajax获得你的任何信息。当然跨域并不完全防止CSRF。</p>
<p><strong>然后我们来考虑一个问题,请求跨域了,那么请求到底发出去没有?</strong><br>请求必然发出去了,但是浏览器拦截了响应。你可能会疑惑明明通过表单的方式可以发起跨域请求,为什么Ajax不会。因为归根结底,<strong>跨域是为了阻止用户读取到另一个域名下的内容,Ajax可以获取响应,浏览器认为这不安全,所以拦截了响应。但是表单并不会获取新的内容,所以可以发起跨域请求。同时也说明了跨域并不能完全阻止CSRF,因为请求已经发出去了</strong></p>
<p>几种常见的方式解决跨域问题</p>
<h3 id="JSONP"><a href="#JSONP" class="headerlink" title="JSONP"></a>JSONP</h3><ol>
<li>原理: 利用script标签发送请求不受同源策略的限制,所以不会产生跨域问题</li>
<li>套路: 动态构建script节点,利用节点的src属性,发送<code>get</code>请求,从而绕开ajax引擎</li>
<li>弊端: <ul>
<li>虽然使用简单且兼容性不错,但只能解决<code>get</code>请求跨域的问题</li>
<li>后端必须配合前端</li>
</ul>
</li>
</ol>
<p>通过<code>&lt;script&gt;</code>标签指向一个需要访问的地址并提供一个回调函数来接收数据当需要通讯时</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">script</span> <span class="attr">src</span>=<span class="string">&quot;http://domain/api?param1=a&amp;param2=b&amp;callback=jsonp&quot;</span>&gt;</span><span class="tag">&lt;/<span class="name">script</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">script</span>&gt;</span></span><br><span class="line"><span class="javascript">  <span class="function"><span class="keyword">function</span> <span class="title">jsonp</span>(<span class="params">data</span>) </span>&#123;</span></span><br><span class="line"><span class="javascript">    <span class="built_in">console</span>.log(data)</span></span><br><span class="line">  &#125;</span><br><span class="line"><span class="tag">&lt;/<span class="name">script</span>&gt;</span></span><br></pre></td></tr></table></figure>
<h3 id="CORS"><a href="#CORS" class="headerlink" title="CORS"></a>CORS</h3><p>CORS需要浏览器和后端同时支持</p>
<p>浏览器会自动进行CORS通信,实现CORS通信的关键是后端。只要后端实现了CORS,就实现了跨域</p>
<p>服务器端设置<code>Access-Control-Allow-Origin</code>就可以开启CORS。该属性表示哪些域名可以访问资源,如果设置<code>通配符*</code>则表示所以网站都可以访问资源</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">res.header(<span class="string">&#x27;Access-Control-Allow-Origin&#x27;</span>, <span class="string">&#x27;允许放行的地址&#x27;</span>)</span><br></pre></td></tr></table></figure>
<p>虽然设置CORS和前端没什么关系,但是通过这种方式解决跨域问题的话,会在发送请求时出现两种情况,分别为<strong>简单请求和复杂请求</strong>。</p>
<h4 id="简单请求"><a href="#简单请求" class="headerlink" title="简单请求"></a>简单请求</h4><p>以Ajax为例,当满足以下条件时,会出发简单请求</p>
<ol>
<li>使用下列方法之一:<ul>
<li>GET</li>
<li>HEAD</li>
<li>POST</li>
</ul>
</li>
<li>Content-Type的值仅限于下列三者之一:<ul>
<li><code>text/plain</code></li>
<li><code>mutatipart/form-data</code></li>
<li><code>application/x-www-form-urlencoded</code></li>
</ul>
</li>
</ol>
<p>请求中的任意 <code>XMLHttpRequestUpload</code>对象均没有注册任何事件监听器;<code>XMLHttpRequestUpload</code>对象可以使用<code>XMLHttpRequest.upload</code>属性访问。</p>
<h4 id="复杂请求"><a href="#复杂请求" class="headerlink" title="复杂请求"></a>复杂请求</h4><p>如果不符合以上条件的请求就肯定是复杂请求了。</p>
<p>对于复杂请求来说,首先会发起一个预检请求,该请求是<code>option</code>方法的,通过该请求来知道服务器端是否允许跨域请求。</p>
<p>对于预检请求来说,如果你使用过Node来设置CORS的话,可能会遇到过坑。</p>
<p>以下以express框架举例:</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">app.use(<span class="function">(<span class="params">req, res, next</span>) =&gt;</span> &#123;</span><br><span class="line">  res.header(<span class="string">&#x27;Access-Control-Allow-Origin&#x27;</span>, <span class="string">&#x27;*&#x27;</span>)</span><br><span class="line">  res.header(<span class="string">&#x27;Access-Control-Allow-Methods&#x27;</span>, <span class="string">&#x27;PUT,GET,POST,DELETE,OPTIONS&#x27;</span>)</span><br><span class="line">  res.header(<span class="string">&#x27;Access-Control-Allow-Headers&#x27;</span>,<span class="string">&#x27;Origin, X-Requested-With, Content-Type, Accept, Authorization, Access-Control-Allow-Credentials&#x27;</span>)</span><br><span class="line"></span><br><span class="line">  next()</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<p>该请求会验证你的<code>Authorization</code>字段,没有的话就会报错</p>
<p>当前端发起了复杂请求后,你会发现就算代码正确,返回的结果也永远报错。因为预检请求也会进入回调中,也会触发<code>next</code>方法,因为预检请求并不包含<code>Authorization</code>字段,所以服务端会报错。</p>
<p>解决这个问题,只需要在回调中过滤<code>option</code>方法即可</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">res.statusCode = <span class="number">204</span></span><br><span class="line">res.setHeader(<span class="string">&#x27;Content-Length&#x27;</span>, <span class="string">&#x27;0&#x27;</span>)</span><br><span class="line">res.end()</span><br></pre></td></tr></table></figure>
<h4 id="postMessage"><a href="#postMessage" class="headerlink" title="postMessage"></a>postMessage</h4><p><code>window.postMessage()</code>方法可以安全的实现跨源通信。</p>
<p><code>window.postMessage()</code>方法提供了一种受控机制来规避同源策略限制,正确使用该方法就很安全</p>
<p>这种方式通常用于获取嵌入页面中的第三方页面数据。一个页面发送消息,另一个页面判断来源并接收消息</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 发送消息端</span></span><br><span class="line"><span class="built_in">window</span>.parent.postMessage(<span class="string">&#x27;message&#x27;</span>, <span class="string">&#x27;http://test.com&#x27;</span>)</span><br><span class="line"><span class="comment">// 接收消息端</span></span><br><span class="line"><span class="keyword">var</span> mc = <span class="keyword">new</span> MessageChannel()</span><br><span class="line">mc.addEventListener(<span class="string">&#x27;message&#x27;</span>, <span class="function"><span class="params">event</span> =&gt;</span> &#123;</span><br><span class="line">  <span class="keyword">var</span> origin = event.origin || event.originalEvent.origin</span><br><span class="line">  <span class="keyword">if</span>(origin === <span class="string">&#x27;http://test.com&#x27;</span>)&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;通过验证&#x27;</span>)</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<h2 id="存储"><a href="#存储" class="headerlink" title="存储"></a>存储</h2><blockquote>
<p>有几种方式可以实现存储功能,分别有什么优缺点?什么是Service Worker?</p>
</blockquote>
<h3 id="cookie-localStorage-sessionStorage-indexDB"><a href="#cookie-localStorage-sessionStorage-indexDB" class="headerlink" title="cookie,localStorage,sessionStorage,indexDB"></a>cookie,localStorage,sessionStorage,indexDB</h3><p>先来通过表格学习这几种存储方式的区别</p>
<table>
<thead>
<tr>
<th>特性</th>
<th align="center">cookie</th>
<th align="right">localStorage</th>
<th align="center">sessionStorage</th>
<th>indexDB</th>
</tr>
</thead>
<tbody><tr>
<td>数据生命周期</td>
<td align="center">一般由服务器生成,可以设置过期时间</td>
<td align="right">除非被清理,否则一直存在</td>
<td align="center">页面关闭就清理</td>
<td>除非被清理,否则一直存在</td>
</tr>
<tr>
<td>数据存储大小</td>
<td align="center">不能超过4K</td>
<td align="right">5M,存储量大</td>
<td align="center">5M,存储量大</td>
<td>无限</td>
</tr>
<tr>
<td>与服务端通信</td>
<td align="center">数据始终在同源的http请求中携带,对于请求性能影响</td>
<td align="right">不参与,仅在本地存储</td>
<td align="center">不参与,仅在本地存储</td>
<td>不参与</td>
</tr>
</tbody></table>
<p>从上表可以看到,<code>cookie</code>已经不建议用于存储。如果没有大量数据存储需求的话,可以使用<code>localStorage</code>和<code>sessionStorage</code>。如果不怎么改变的数据尽量使用<code>localStorage</code>,否则用<code>sessionStorage</code>存储。</p>
<h2 id="Service-Worker"><a href="#Service-Worker" class="headerlink" title="Service Worker"></a>Service Worker</h2><p>Service Worker是运行在浏览器背后的<strong>独立线程</strong>,一般可以用来实现缓存功能。使用Service Worker的话,传输协议必须为<strong>HTTPS</strong>,因为Service Worker中涉及到请求拦截,所以必须使用<code>HTTPS</code>协议保障安全。</p>
<p>Service Worker实现缓存功能一般分为三个步骤: 首先需要注册Service Worker,然后监听到<code>install</code>事件后就可以缓存需要的文件,那么在下次用户访问的时候就可以通过拦截请求的方式查询是否存在缓存,存在缓存的话就可以直接读取缓存文件,否则就去请求数据。以下是这个步骤的实现:</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// index.js</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(navigator.serviceWorker) &#123;</span><br><span class="line">  navigator.serviceWorker</span><br><span class="line">  <span class="comment">// register方法接收两个参数,第一个是service Worker文件的路径,第二个参数是Service Worker的配置项,可选填,其中比较重要的是scope属性。register返回一个Promise。如果注册失败,可以通过.catch()捕获错误信息;如果注册成功可以通过.then()来获取一个ServiceWorkerRegistration 的实例</span></span><br><span class="line">    .register(<span class="string">&#x27;sw.js&#x27;</span>)</span><br><span class="line">    .then(<span class="function"><span class="keyword">function</span>(<span class="params">registration</span>) </span>&#123;</span><br><span class="line">      <span class="built_in">console</span>.log(<span class="string">&#x27;Service Worker注册成功&#x27;</span>)</span><br><span class="line">    &#125;)</span><br><span class="line">    .catch(<span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">      <span class="built_in">console</span>.log(<span class="string">&#x27;service worker注册失败&#x27;</span>)</span><br><span class="line">    &#125;)</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// sw.js</span></span><br><span class="line"><span class="comment">// 监听install事件,回调中缓存所需文件</span></span><br><span class="line">self.addEventListener(<span class="string">&#x27;install&#x27;</span>, <span class="function"><span class="params">e</span> =&gt;</span> &#123;</span><br><span class="line">  e.waitUntil(</span><br><span class="line">    caches.open(<span class="string">&#x27;my-cache&#x27;</span>).then(<span class="function"><span class="keyword">function</span>(<span class="params">cache</span>) </span>&#123;</span><br><span class="line">      <span class="keyword">return</span> cache.addAll([<span class="string">&#x27;./index.html&#x27;</span>, <span class="string">&#x27;./index.js&#x27;</span>])</span><br><span class="line">    &#125;)</span><br><span class="line">  )</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 拦截所有请求事件</span></span><br><span class="line"><span class="comment">// 如果缓存中已经由请求的数据就直接缓存,否则去请求数据</span></span><br><span class="line">self.addEventListener(<span class="string">&#x27;fetch&#x27;</span>, <span class="function"><span class="params">e</span> =&gt;</span> &#123;</span><br><span class="line">  e.responseWith(</span><br><span class="line">    caches.match(e.request).then(<span class="function"><span class="keyword">function</span>(<span class="params">response</span>) </span>&#123;</span><br><span class="line">      <span class="keyword">if</span>(response) &#123;</span><br><span class="line">        <span class="keyword">return</span> response</span><br><span class="line">      &#125;</span><br><span class="line">      <span class="built_in">console</span>.log(<span class="string">&#x27;fetch source&#x27;</span>)</span><br><span class="line">    &#125;)</span><br><span class="line">  )</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
                
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